Underreamers are typically used to enlarge the diameter of a bore hole, for one or more of a variety of reasons. It is often necessary for the underreamer to first travel through a casing(s), having a diameter smaller than the diameter desired down-hole of the casing. Accordingly, underreamers are provided with cutting arms that may be retracted during travel through the casing. When a predetermined depth is reached, the cutting arms are actuated to an extended position, and drilling with the underreamer commences. Before an underreamer is brought into service on each occasion, it may be necessary that each arm be locked distinctly in the inactive position. This is to prevent the arms from being deployed unintentionally following variations in the pressure of the fluid passing through the underreamer, until particular time and/or depth chosen by the operator. In particular therefore, for each new use of a typical underreamer, it is often necessary to remove each arm on each occasion, and possibly the housing thereof, in order to renew the distinct locking means.
In addition, this type of equipment is subjected to very harsh forces under working conditions that are known to be very difficult and therefore very expensive. Firstly, an equipment breakdown may cost significant time, money and resources in attempting to save the equipment, for example jammed at a great depth, and in particular saving the bore hole made at great expense and which, otherwise, could be definitively condemned. Secondly, when the equipment is recovered, an equipment breakdown must be able to be repaired very easily because the technical repair means available on or close to a drilling platform are sometimes limited.